1. Field of the Invention
The invention relates to a memory device and a method for storing information in the memory device. More specifically, the invention relates to a memory device using a threshold voltage switching device as a switching element and a method for storing information in the memory device.
2. Related Technology
Magnetic, optical, and electric recordings are widely used for recording, storing, and reproducing information. In particular, electric recording has no limitation on the size of media for information storage via current flow, thus enabling a significant increase in storage density. Media for electric recording include dynamic random access memories (DRAMs), flash memory devices for storing information via an injection of charges on a silicon oxide nitride (SiNxOy) layer, and ferroelectric random access memories (FRAMs) utilizing dielectric polarization of ferroelectrics. Semiconductor memories have advantages of compact size, high reliability, and low fabrication costs. In addition, since semiconductor memories have an advantage of high speed operation, they are widely utilized in applications including main memories in computers, embedded memories, and cache memories in microprocessors.
DRAM, which is most generally used in semiconductor memories, includes a plurality of unit cells each having a word line operated by a row address, a bit line operated by a column address, a cell transistor whose drain and gate are connected to the bit line and the word line, respectively, and a capacitor for charge storage connected to a source of the cell transistor.
Hereinafter, a mechanism associated with a writing operation for storing information in the DRAM cell and a reading operation for reading the information will be described.
When a word line is activated, a cell transistor connected to the word line turns on. While the voltage of the bit line is applied to a unit cell through a drain of the cell transistor, an electric charge is stored in the storage electrode of the capacitor. At this time, the voltage supplied into the bit line is 0V or Vdd (i.e., an operation voltage). A fixed power voltage (Vcc), which is generally about half of the operation voltage (Vdd), is supplied to a plate electrode of the capacitor.
With trends toward more highly integrated semiconductor memory devices, there has been a reduction in memory device size. In an attempt to form a DRAM capacitor having a maximum capacitance within a limited area, the formation of a trench-type storage electrode or the application of a dielectric film having a high dielectric constant have been suggested. For transistors, there have been active efforts (e.g., the formation of a recessed channel) to ensure a sufficient channel length within a small area. Despite these attempts, there is no memory device that completely satisfies the requirements of large capacity, high integration, and high speed. Accordingly, there still remains a demand for memory devices that can be substituted for transistors, occupy a small area, and are readily manufactured via a simplified procedure.